Batteries
The first battery was created by Alessandro Volta in 1800. To create his battery, he made a
stack by alternating layers of zinc, blotting paper soaked in salt water, and silver, like this:
This arrangement was known as a voltaic pile. The top and bottom layers of the pile must
be different metals, as shown. If you attach a wire to the top and bottom of the pile, you can
measure a voltage and a current from the pile. The pile can be stacked as high as you like,
and each layer will increase the voltage by a fixed amount.
In the 1800s, before the invention of the electrical generator (the generator was not invented
and perfected until the 1870s), the Daniell cell (which is also known by three other names -the "Crowfoot cell" because of the typical shape of the zinc electrode, the "gravity cell"
because gravity keeps the two sulfates separated, and a "wet cell," as opposed to the
modern "dry cell," because it uses liquids for the electrolytes), was extremely common for
operating telegraphs and doorbells. The Daniell cell is a wet cell consisting of copper and
zinc plates and copper and zinc sulphates.
To make the Daniell cell, the copper plate is placed at the bottom of a glass jar. Copper
sulfate solution is poured over the plate to half-fill the jar. Then a zinc plate is hung in the jar
as shown and a zinc sulfate solution poured very carefully into the jar. Copper sulfate is
denser than zinc sulfate, so the zinc sulfate "floats" on top of the copper sulfate. Obviously,
this arrangement does not work very well in a flashlight, but it works fine for stationary
applications. If you have access to zinc sulfate and copper sulfate, you can try making your
own Daniell cell.
Experiments
If you want to learn about the electrochemical reactions used to create batteries, it is easy to
do experiments at home to try out different combinations. To do these experiments
accurately, you will want to purchase an inexpensive ($10 to $20) volt-ohm meter at the
local electronics or hardware store. Make sure that the meter can read low voltages (in the 1volt range) and low currents (in the 5- to 10-milliamp range). This way, you will be able to see
exactly what your battery is doing.
You can create your own voltaic pile using coins and paper towels. Mix salt with water (as
much salt as the water will hold) and soak the paper towel in this brine. Then create a pile by
alternating pennies and nickles. See what kind of voltage and current the pile produces. Try
a different number of layers and see what effect it has on voltage. Then try alternating
pennies and dimes and see what happens. Also try dimes and nickels. Other metals to try
include aluminum foil and steel. Each metallic combination should produce a slightly different
voltage.
Another simple experiment you can try involves a baby food jar (if you don't have a baby
around the house, just purchase a few jars of baby food at the market and empty them out),
a dilute acid, wire and nails. Fill the jar with lemon juice or vinegar (dilute acids) and place a
nail and a piece of copper wire in the jar so that they are not touching. Try zinc-coated
(galvanized) nails and plain iron nails. Then measure the voltage and current by attaching
your volt meter to the two pieces of metal. Replace the lemon juice with salt water, and try
different coins and metals as well to see the effect on voltage and current.
Probably the simplest battery you can create is called a zinc/carbon battery. By
understanding the chemical reaction going on inside this battery, you can understand how
batteries work in general.
Imagine that you have a jar of sulfuric acid (H2SO4). Stick a zinc rod in it, and the acid will
immediately start to eat away at the zinc. You will see hydrogen gas bubbles forming on the
zinc, and the rod and acid will start to heat up. Here's what is happening:
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The acid molecules break up into three ions: two H+ ions and one SO4-- ion.
The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to become Zn++
ions.
The Zn++ ions combine with the SO4-- ion to create ZnSO4, which dissolves in the
acid.
The electrons from the zinc atoms combine with the hydrogen ions in the acid to
create H2 molecules (hydrogen gas). We see the hydrogen gas as bubbles forming
on the zinc rod.
If you now stick a carbon rod in the acid, the acid does nothing to it. But if you connect a wire
between the zinc rod and the carbon rod, two things change:
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The electrons flow through the wire and combine with hydrogen on the carbon rod,
so hydrogen gas begins bubbling off the carbon rod.
There is less heat. You can power a light bulb or similar load using the electrons
flowing through the wire, and you can measure a voltage and current in the wire.
Some of the heat energy is turned into electron motion.
The electrons go to the trouble to move to the carbon rod because they find it easier to
combine with hydrogen there. There is a characteristic voltage in the cell of 0.76 volts.
Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up
and the battery "dies."